Aztreonam is a monobactam antibiotic, having the chemical name [3S-[3α(Z),4β]]-3-[[(2-amino-4-thiazolyl)[(1-carboxy-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidinesulfonic acid, and the following structure:

U.S. Pat. No. 4,775,670 discloses a process for acylating a compound of formula:
The acylation comprises reacting the compound with a carboxylic acid or the corresponding carboxylic acid halide or carboxylic acid anhydride (R1—OH) in the presence of a carbodiimide such as dicyclohexylcarbodiimide and a substance capable of forming an active ester in situ, such as N-hydroxybenzotriazole. The deprotection in U.S. Pat. No. 4,775,670 is carried out by reaction of the acylation product with trifluoroacetic acid in the presence of anisole under anhydrous conditions. The reagents used in this process are toxic and expensive.
U.S. Pat. No. 4,946,838 discloses an alternative process for making crystalline anhydrous aztreonam and a crystalline anhydrous form of the antibacterial agent. The synthetic path described in this patent is complicated, including, for example, the hydrolysis of a diphenylester protecting group with trifluoracetic acid in anisole under anhydrous conditions to produce the α-form of aztreonam. The α-form is recrystallized from an anhydrous organic solvent to produce the β-form of aztreonam.
U.S. Pat. No. 5,254,681 discloses a process for preparing aztreonam that comprises acylating azetidine with 2-(2-amino-4-thiazolyl)-2-(Z)-(alkoxyimino)acetic acid in the presence of 1-hydroxy-benzotriazole and dicyclohexylcarbodiimide. The reagents used in this process are toxic and expensive, and their disposal is difficult.
PCT Publication No. WO 2004/013133 discloses a process for preparing aztreonam by hydrolyzing the ester group of [3S-[3α(Z),4β]]-3-[[(2-amino-4-thiazolyl)[(1-t-butoxycarbonyl-1-methylethoxy)imino]acetyl]amino]-4-methyl-2-oxo-1-azetidinesulfonic acid (t-Bu aztreonam) by reacting the ester with an aqueous acid, at elevated temperatures. The application also describes the condensation of azetidine and TAEM with acetonitrile and a base, optionally in the presence of other solvents such as THF, to obtain t-Bu aztreonam.
A further simplification of the process would be desirable. The present invention provides such a simplified process.